In a background art, according to an illuminating optical system mounted to an endoscope apparatus, there is frequently used a light guide constituted by an optical fiber in which a refractive index of a core is constant regardless of a portion thereof, however, a light guide of this type poses a problem that a luminous intensity distribution characteristic thereof cannot be adapted to an endoscope having a wide angle object lens, that is, an intensity of a light ray emitted to a peripheral portion having a large angle from an optical axis is more attenuated than an intensity of a light ray emitted to a center portion.
For example, FIG. 3 shows a luminous intensity distribution (indicating rates of light rays emitted by respective angles relative to an optical axis by constituting a reference (100%) of an intensity of an emitted light ray in parallel with the optical axis) of a light guide in which a refractive index of a core is 1.62, a refractive index of a clad is 1.48 and a numerical aperture NA is about 0.66. When the numerical aperture NA is converted into the angle from the optical axis, the angle becomes about 41.2 degree, as shown by FIG. 3, an emitted light ray having a large angle from the optical axis is more attenuated than a light ray having a small angle for the reasons, for example, a long distance of passing inside of the optical fiber, and even when the angle from the optical axis is at a vicinity of 20 degrees, the luminous intensity is reduced to be halved.
The object lens used in an endoscope is frequently provided with a wide angle in which an angle of view (viewing angle) exceeds 100 degrees (semi angle of view is 50 degrees), in a case of illumination by the above-described luminous intensity distribution, an image at a peripheral field of view region becomes dark and therefore, it is difficult to carry out sufficient observation. Further, although there is also known an illuminating optical system widening an illuminating angle by arranging a concave lens at a front end portion of a light guide, only a light amount at a peripheral portion which is inherently liable to be insufficient is scattered and it is not improved that the light amount of the peripheral portion is smaller than that of a center portion thereof.
On the other hand, there is proposed an endoscope illuminating optical system having a wider angle having a refractive indexes distribution type lens in which a refractive index is increased as being proximate to a center portion from a peripheral portion (refer to JP-A-10-54945).